Tubular polymerization reactors and polymers made therein

ABSTRACT

Tubular reactor apparatus and processes are provided for improved polymerization including using chain transfer agents and multiple monomer feeds spaced lengthwise along the tubular reactor providing high conversions of monomer into polymer. The invention also relates to polymers made from such a tubular reactor apparatus and processes including those polymers having a low haze value, a density over 0.92 g/cm 3  and/or having terminal carbonyl groups. The apparatus and methods uncouple or reduce the dependency between the monomer concentration and transfer agent concentration. The uncoupling in other embodiments may also be varied leading to multiple desirable effects.

FIELD OF THE INVENTION

This invention relates to an apparatus and processes for improvedpolymerization in tubular polymerization reactors, including those usingchain transfer agents and multiple monomer feeds spaced lengthwise alongthe tubular reactor, to provide high conversions of monomer intopolymer. The invention also relates to polymers made from such processesand apparatus, including those polymers having a low haze value, adensity over 0.92 g/cm³ and/or having terminal carbonyl groups.

BACKGROUND

Tubular polymerization reactor apparatus is used to make polyethylene,mainly by free radical initiation. Initiators may be oxygen, peroxidesand similar agents. Catalysts used for coordination polymerization mayalso, where appropriate, be used.

The highly exothermic polymerization reaction is performed in a tubularreactor (“tube”) forming part of the apparatus under high reactoroperating pressure (2000 bar to 3500 bar) under turbulent flow, at hightemperatures (120° C. to 330° C.). Heat may be removed through the tubewall, which may be liquid cooled. Tubular reactors may have outputs,which vary from 50 kT to 400 kT per annum. Low cost production requiresa high conversion of monomers to give as large an output of commerciallydesirable polymer types as is possible from given investment.

Referring to FIG. 1, a tubular polymerization reactor 100 has a tube 2with a length typically from 200 to 1600 meters determined on the basisof the desired heat removal capacity and a diameter of from 20 to 100 mmdetermined on the basis of the desired capacity and the requiredturbulent flow.

A medium pressure, primary compressor 4, which may include a number ofcompressor stages, not individually shown, is connected at its intakeside to a source of fresh ethylene supplied by a conduit 6, and recycledethylene from a recycle conduit 8 at a pressure of from 20 to 70 bar.The primary compressor raises the pressure of the monomer on the outletside to a pressure of from 250 bar to 350 bar. A high pressure,secondary compressor 5, which may include a number of compressor stages,is connected at its intake side to the outlet side of the primarycompressor 4 and raises the pressure of the feed containing ethylenefurther to the reactor operating pressure as indicated above of from2000 to 3500 bar. The compressed pressurized monomer is then fed throughconduits 12, 14 to various monomer feed locations 3 spaced lengthwisealong tube 2.

Multiple free-radical initiator or catalyst injection positions 7 arealso spaced lengthwise of the tube 2 to cause the monomer to beconverted into polymer in at least two reaction zones formed inside thetube 2.

A mixture of polymer and unreacted monomer formed in the tube 2 passesfrom tube outlet 16 to a separating and recycling part of thepolymerization apparatus. This part includes a high-pressure separator(HPS) 18, which receives the monomer-polymer mixture from the outlet ofthe tube 2. The HPS is connected to convey a mixture of polymer andmonomer produced, to a low-pressure separator (LPS) 20 for furthermonomer removal. The resulting molten polymer phase is passed from theLPS 20 to a polymer finishing section with an extruder 22. A volatilemonomer-rich phase comprising unreacted monomer separated in HPS 18,passes through a recycle conduit 24 at a pressure of approximately thatof the outlet of the primary compressor 4 through line 26 to join themonomer containing feed passing from the primary to the secondarycompressor 5. The volatile monomer rich phase including unreactedmonomer from the LPS 20 passes to a low pressure purge compressor 21,which may have a number of stages, at a pressure above that at theintake of the primary compressor to the intake of the primary compressor4.

At some location in the circuit a chain transfer agent is added forsupply to the tube 2. Transfer agents are used to reduce the molecularweight, which can be expressed in a melt index (MI) value, and to narrowthe molecular weight distribution (MWD).

A typical product range is shown in FIG. 2 and covers a melt index(“MI”, I_(2.16)) of from 0.1 to 50 dg/min, a molecular weightdistribution (MWD) of from 5 to 50 and a haze of from 1 to 20. Haze isdetermined by ASTM D-1003; MI is determined by ASTM-1238 Condition E; Mwand Mn were measured by GPC (Gel Permeation Chromatography) on a Waters150 gel permeation chromatograph equipped with a differential refractiveindex (DRI) detector and a Chromatix KMX-6 on line light scatteringphotometer. The system was used at 135° C. with 1,2,4-trichlorobenzeneas the mobile phase. Shodex (Showa Denko America, Inc) polystyrene gelcolumns 802, 803, 804 and 805 were used. This technique is discussed in“Liquid Chromatography of Polymers and Related Materials III”, J. Cazeseditor, Marcel Dekker. 1981, p. 207, which is incorporated herein byreference. No corrections for column spreading were applied; however,data on generally accepted standards, e.g., National Bureau of StandardsPolyethylene 1484, and anionically produced hydrogenated polyisoprenes(an alternating ethylene-propylene copolymer) demonstrated that suchcorrections on Mw/Mn (=MWD) were less than 0.05 units. Mw/Mn wascalculated from elution times. The numerical analyses were performedusing the commercially available Beckman/CIS customized LALLS softwarein conjunction with the standard Gel Permeation package. Calculationsinvolved in the characterization of polymers by ¹³CNMR follow the workof F. A. Bovey in “Polymer Conformation and Configuration” AcademicPress, New York, 1969.

In practical use of the apparatus, product quality has to be balancedwith desired production economics. Higher conversion (giving low energyand recycle costs) tends to lead to a broader MWD and significantbranching which leads to high and unacceptable haze values. Low densitypolyethylene requires production of relatively many short chainbranches. Olefinically unsaturated comonomers are then used which have alow transfer coefficient (efficiency of transfer agents) and hencelittle chain length reducing activity. Examples are propylene orbutene-1. A high concentration of such comonomers is needed to achieve adesired melt index, restricting the productive capacity on a givenreactor. In some cases, certain areas of theoretically available MI,haze and density combinations cannot be produced at an acceptable cost.Particularly narrow molecular weight distribution (MWD), relatively highdensity polyethylenes generally cannot be made economically withsaturated alkane transfer agents (which do not incorporate in the chain)as they have a very low transfer constant, lower than the propylene andbutene-1 used for low density polyethylenes.

An initiator or catalyst injection position is associated with eachreaction zone. Injection of the initiator causes an exothermictemperature rise which is removed by a cooling at the zone anddownstream of that zone. The cooling is effected through the tube wall,optionally aided by a cooling liquid as a heat transfer medium and/or bya feed of cold monomer that is added downstream. Further, initiator maybe added downstream to form another reaction zone for convertingadditional monomer into polymer.

Generally speaking, in the prior art, transfer agents have been added soas to have roughly the same concentration of chain transfer agent ineach monomer feed. From an apparatus point of view, this can be achievedby mixing the transfer agent with the monomer fed before the monomer iscompressed by the secondary compressor. The transfer agent is then addedequally along the length of the tube, although it may be consumedunequally and so concentration variations along the tube may result.

In FIG. 1, a source 23 of transfer agent is connected to the intake ofthe primary compressor 4 and hence distributed, after passing throughthe secondary compressor 5, to the different monomer feeds 3 spacedalong the tube 2. The recycle stream 8 coming from the LPS 20 and thepurge compressor 21 is also passed to the intake of primary compressor4. The recycle from the HPS 18 which contains unconsumed transfer agentis passed to the intake of the secondary compressor. Thus the transferagent and monomer form a single, common gas stream with the desiredconcentration of transfer agent for compression in the secondarycompressor 5 and for supply to the various feed positions 3 along thetube 2.

Furthermore, by selecting a transfer agent which has a low chaintransfer activity, higher concentrations of transfer agent have to beused in the non-polymer gaseous fraction of the tube contents to achievea target MI. The low chain transfer activity contributes to the creationof small transfer agent concentrations along the length of the tubewhere the chain transfer agents also have a low reactivity ratio. Byusing unsaturated transfer agents with low chain transfer activity,branches are formed along the polymer backbone and the density of theresulting polymer is reduced. In such apparatus, mostly chain transferagents have been used having a chain transfer constant of less than0.03.

The recycle from the HPS and LPS still contain, for transfer agents witha low reactivity ratio, a high level of transfer agent and the amountthat is added from the source 23 is low relative to that present in therecycle 26 and 8.

It would be desirable to have methods and apparatus for transfer agentaddition and selection so as to increase the process efficiency whileobtaining a satisfactory commercial product, or to produce moresatisfactory commercial products at prevailing process efficiencies.

SUMMARY

The present invention provides apparatus and methods whichadvantageously uncouple, or reduce the dependency, between the monomerconcentration and transfer agent concentration and permit theseconcentrations to be varied along the tube length.

In one embodiment the invention provides a tubular polymerizationreactor apparatus including a source of fresh monomer, first and secondcompressor stages for compressing monomer, a reactor tube, multiplefeeds spaced lengthwise along the reactor tube for supplying monomer tothe reactor, multiple free-radical or catalyst injection positionsspaced lengthwise along the reactor tube for causing monomer to beconverted into polymer inside the reactor, separators for receiving amonomer-polymer mixture from the reactor tube and separating the mixtureinto a volatile monomer-rich phase and molten polymerization phase,conduits for recycling the monomer-rich phase to the first and/or secondcompressor stages for recycling unreacted monomer to the reactor tube,and a source of transfer agent for modifying the molecular weight of thepolymer for compression and feeding to the reactor tube. Compressormeans is provided for compressing a transfer agent rich streamseparately from a transfer agent-poor monomer stream and means isprovided for feeding the compressed transfer agent rich stream to apolymerization reaction zone upstream of at least one reaction zonereceiving the transfer agent-poor stream containing less than 30 wt. %of the transfer agent relative to the transfer agent rich stream.

In another embodiment the invention provides a process for thepolymerization of ethylene. The process includes combining fresh monomerand recycled monomer; compressing the combined monomer; supplying themonomer using multiple feeds to multiple reaction zones in a tubularreactor for polymerization by a free radical initiator to form amonomer-polymer mixture; separating the mixture into a volatilemonomer-rich phase and molten polymer-rich phase; recycling forcompression the monomer-rich phase and supplying the monomer-rich phaseto the reactor; and introducing transfer agent into the reactor tomodify the molecular weight of the polymer. The transfer agent includesa chain terminating transfer agent having a transfer coefficient ofgreater than 0.01, the transfer agent being in a transfer agent richstream separately from a transfer agent-poor monomer stream of thetransfer agent, to a polymerization reaction zone upstream of at leastone downstream reaction zone receiving the transfer agent-poor streamhaving less than 30% of the transfer agent relative to the transferagent rich stream so as to achieve a depletion in the concentration ofthe transfer agent in the downstream reaction zone.

BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription, appended claims, and accompanying drawings, where:

FIG. 1 shows a tubular polymerization reactor.

FIG. 2 shows a typical polyethylene product range and covers a meltindex (MI) of from 0.1 to 50, a molecular weight distribution (MWD) offrom 5 to 50 and a haze of from 1 to 20.

FIG. 3 shows one embodiment of a reactor apparatus of the invention.

DETAILED DESCRIPTION

In one embodiment, and referring now to FIG. 3, the present inventionprovides a tubular polymerization reactor apparatus 200 having:

a source of fresh monomer 6, unrecycled, generally not containingtransfer agent;

medium and high pressure compressors 4 and 5, respectively, forcompressing monomer;

a reactor tube 2;

multiple monomer feed 3, spaced lengthwise along the reactor tube 2 forsupplying monomer to the tube;

multiple free-radical or catalyst injection positions 7 spacedlengthwise along the tube 2 for causing monomer to be converted intopolymer inside the tube in a reaction zone (not shown);

separators 18 and 20 for receiving a monomer-polymer mixture from thereactor tube 2 and separating the mixture into a volatile monomer-richphase and molten, polymer-rich phase;

conduits 8 and 26 for recycling the monomer-rich phase for compressionand for recycling unreacted monomer or monomers to the tubular reactor;and

a source of transfer agent 30 for modifying the molecular weight of thepolymer including means 32 for compressing the transfer agent andfeeding it to the tubular reactor via one or more transfer agent feeds34 separately from the monomer feed(s) 3.

In the conventional apparatus described above with reference to FIG. 1,transfer agent and monomer were mixed before a final common compressionstep and so supplied at equal transfer agent/monomer ratios at thedifferent feeds 3. By contrast, in embodiments of the invention,compressor means 32 is provided for compressing a transfer agent richgas stream 30 separately from a transfer agent-poor gas stream 12, and atransfer agent feeds 34 are provided for feeding the compressed transferagent-rich stream to a polymerization reaction zone upstream of at leastone reaction zone receiving the transfer agent-poor stream. As a resultless transfer agent is supplied towards the downstream end of thetubular reactor, i.e., the part of the reactor containing one or morereaction zones downstream of one or more reaction zones located towardsthe upstream end of the reactor than is supplied towards those upstreamreaction zone or zones.

Examples of chain transfer agents include ethylene, tetramethylsilane,cyclopropane, sulfur hexafluoride, methane, t-butanol, perfluoropropane,deuterobenzene, ethane, ethylene oxide, 2,2-dimethylpropane, benzene,dimethyl sulfoxide, vinyl methyl ether, methanol, propane,2-methyl-3-buten-2-ol, methyl acetate, t-butyl acetate, methyl formate,ethyl acetate, butane, triphenylphosphine, methylamine, methyl benzoate,ethyl benzoate, N,N-diisopropylacetamide, 2,2,4-trimethylpentane,n-hexane, isobutane, dimethoxymethane, ethanol, n-heptane, n-butylacetate, cyclohexane, methylcyclohexane, 1,2-dichlorethane,acetronitrile, N-ethylacetamide, propylene, n-decane,N,N-diethylacetamide, cyclopentane, acetic anhydride, n-tridecane,n-butyl benzoate, isopropanol, toluene, hydrogen, acetone,4,4-dimethylpentene-1, trimethylamine, N,N-dimethylacetamide,isobutylene, n-butyl isocyanate, methyl butyrate, n-butylamine,N,N-dimethylformamide, diethyl sulfide, diisobutylene, tetrahydrofuran,4-methylpentene-1, p-xylene, p-dioxane, trimethylamine, butene-2,1-bromo-2-chlorethane, octene-1, 2-methylbutene-2, cumene, butene-1,methyl vinyl sulfide, n-butyronitrile, 2-methylbutene-1, ethylbenzene,n-hexadecene, 2-butanone, n-butyl isothiocyanate, methyl3-cyanopropionate, tri-n-butylamine, 3-methyl-2-butanone,isobutyronitrile, di-n-butylamine, methyl chloroacetate,3-methylbutene-1,2-dibromoethane, dimethylamine, benzaldehyde,chloroform, 2-ethylhexene-1, propionaldehyde, 1,4 dichlorobutene-2,tri-n-butylphosphine, dimethylphosphine, methyl cyanoacetate, carbontetrachloride, bromotrichloromethane, di-n-butylphosphine, andphosphine.

For further details of transfer agents, see Advances In Polymer Science,Vol. 7, p. 386-448 (1970). Table 7 therein ranks several transfer agentsin order of the chain transfer constant determined under set conditions.The tendency to copolymerize is indicated by the reactivity, alsodetermined under set conditions.

Typical monomers include: vinyl ethers such as vinyl methyl ether, vinyln-butyl ether, vinyl phenyl ether, vinyl beta-hydroxy-ethyl ether, andvinyl dimethylamino-ethyl ether; olefins such as ethylene, propylene,butene-1, cis-butene-2, trans-butene-2, isobutylene,3,3,-dimethylbutene-1, 4-methylpentene-1, octene-1, and styrene; vinyltype-esters such as vinyl acetate, vinyl butyrate, vinyl pivalate, andvinylene carbonate; haloolefins such as vinyl fluoride, vinylidenefluoride, tetrafluoroethylene, vinyl chloride, vinylidene chloride,tetrachloroethylene, and chlorotrifluoroethylene; acrylic-type esterssuch as methyl acrylate, ethyl acrylate, n-butyl acrylate, t-butylacrylate, 2-ethylhexyl acrylate, alpha-cyanoisopropyl acrylate,beta-cyanoethyl acrylate, o-(3-phenylpropan-1,3,-dionyl)phenyl acrylate,methyl methacrylate, n-butyl methacrylate, t-butyl methacrylate,cyclohexyl methacrylate, 2-ethylhexyl methacrylate, methyl methacrylate,glycidyl methacrylate, beta-hydroxethyl methacrylate, beta-hydroxpropylmethacrylate, 3-hydroxy-4-carbomethoxy-phenyl methacrylate,N,N-dimethylaminoethyl methacrylate, t-butylaminoethyl methacrylate,2-(1-aziridinyl)ethyl methacrylate, diethyl fumarate, diethyl maleate,and methyl crotonate; other acrylic-type derivatives such as acrylicacid, methacrylic acid, crotonic acid, maleic acid, methyl hydroxy,maleate, itaconic acid, acrylonitrile, fumaronitrile,N,N-dimethylacrylamide, N-isopropylacrylamide, N-t-butylacrylamide,N-phenylacrylamide, diacetone acrylamide, methacrylamide,N-phenylmethacrylamide, N-ethylmaleimide, and maleic anhydride; andother compounds such as allyl alcohol, vinyltrimethylsilane,vinyltriethoxysilane, N-vinylcarbazole, N-vinyl-N-methylacetamide,vinyldibutylphosphine oxide, vinyldiphenylphosphine oxide,bis-(2-chloroethyl) vinylphosphonate and vinyl methyl sulfide. Themonomers/transfer agents are ranked in reactivity ratio order in Table10. See id.

The difference in the concentration in mol % of transfer agent suppliedto the successive reaction zones may be in excess of 20%. A simpleconstructional option for implementing a difference of such a magnitudeis providing conduits for compressing some or all of the transfer agentwhich are separate from other conduits for compressing monomer.Therefore compressor means 32 may include a further compressor stage foran initial compression of the transfer agent obtained from the source oftransfer agent 30, and a section of the second compressor stageconnected to the outlet of the further compressor stage for raising thegas stream containing transfer agent to a pressure suitable forsupplying to the reactor, the section being optionally used additionallyfor compressing part of the monomer obtained from the outlet of thefirst compressor stage, the transfer agent being passed through conduitssegregated from another section of the second compressor stage which isused for compressing the transfer agent-poor monomer stream. In thismanner fresh transfer agent is concentrated, to have effect in theupstream reaction zone.

The transfer agent rich stream may be connected to be introducedupstream of all reaction zones receiving a transfer agent-poor stream.This may magnify the effect and benefit of embodiments of the invention.In another embodiment, where three or more reaction zones are used, thetransfer agent rich stream may be supplied to a reaction zoneintermediate two reaction zones receiving a transfer agent poor-stream.This may tend to dilute the benefit of upstream transfer agentinjection.

An added modification in another embodiment is to further uncouple thevolatile part of the mixture from the tubular reactor outlet from thegeneral monomer feed and to not distribute the residual transfer agentpresent therein to the feed streams used for the downstream reactionzones. The monomer-rich recycled stream from the combined separators isconnected to be supplied to an extent exceeding from 75% by volume ofthe total recycle stream mass to one or more reaction zones upstream ofat least one downstream reaction zone. The remaining part of the recyclestream mass can be combined with the monomer feed supplied to the mediumpressure compressor intake for distribution to all other feed positions.The effect can be optimized by providing a recycled monomer-rich feedconnected to be supplied to an extent of from 75 to 100% of its volumeto a first reaction zone located furthest upstream of all other reactionzones spaced lengthwise along the tubular reactor.

Construction of an apparatus for providing such an arrangement resultswhere the transfer agent-rich stream is connected to be passed from thefurther compressor stage and combined with the recycled monomer-richstream for compression in the segregated section of the secondcompressor stage.

While the monomer can be any molecule or molecules capable of additionpolymerization by either a free-radical mechanism or coordinationcatalytic mechanism, the predominant monomer may be ethylene. Othermonomers which incorporate less easily and may have transfer-activityand a molecular weight limiting effect (and indeed can, for somepurposes, be regarded as incorporating transfer agents) include: vinylacetate, ethyl acrylate, methyl acrylate, butyl acrylate, and the like.Most commonly ethylene is used at a mole concentration of at least 90%,or 96%, or 98%, the percentages being based on the total weight of allmonomer and transfer agent present.

While, in theory, Ziegler-Natta catalysts can be used such as TiCl₃based catalysts with an aluminum alkyl activator, or metallocenecatalysts with an alumoxane or non-coordinating anion activator, orusing a free-radical initiator, generally initiators can be selectedfrom the list given in Advances of Polymer Science op cit.

Preferably, the transfer agents have the properties:

1) they possess high transfer activity which enables them to reduce themelt index (MI) at relatively low concentrations of transfer agents. Areview of the transfer agents in Advances of Polymer Science op cit willshow the suitable candidates. Particularly suited are methyl vinylsulfide, and n-butyronitrile; and

2) they incorporate and have a high reactivity ratio so theconcentration will deplete, in the absence of further additiondownstream, as the reaction mixture passes downstream in the tubethrough the successive reaction zones, with the proviso that suchtransfer agents to do not lower the density and incorporate at the endsof the polymer chain without creating branches.

Other transfer agents may be present in addition, to form short-chainbranches to the extent that the desired conversion enhancing effect isnot negated.

Operating conditions for tubular reactors are in general well known, butthe amount of monomer vs. transfer agent fed can advantageously bebiased to achieve a high transfer agent concentration in the upstreamfeed or feeds.

The addition of the free-radical initiator to a monomer mixture causesgenerally the formation of a reaction zone in which monomer is convertedexothermally and the resulting temperature rise is controlled by coolingand addition of further monomer to the downstream end of the reactionzone. Thus, temperature and reagent concentration can vary along thelength of the tubular reactor.

Embodiments of the invention generally provide, for this aspect, aprocess for polymerization of ethylene which includes combining freshmonomer and recycled monomer and compressing the combined recycled andfresh monomer and supplying the monomer using multiple feeds to multiplereaction zones in a tubular reactor for polymerization by a free radicalinitiator to form a monomer-polymer mixture; and separating the mixtureinto a volatile monomer-rich phase and molten polymer-rich phase withthe monomer-rich phase being recycled for compression and supplying tothe reactor, the process further including introducing transfer agentinto the reactor to modify the molecular weight of the polymer.

In such embodiments, the transfer agent includes a chain terminatingtransfer agent of a carbonyl containing linear compound incorporatingpredominantly in a position close to the end of the polymer chain,having a transfer coefficient of greater than 0.01, the transfer agentbeing in a transfer agent rich stream separately from a transferagent-poor monomer stream to a polymerization reaction zone upstream ofat least one reaction zone receiving the transfer agent-poor stream.

As indicated above, the chain terminating transfer agent rich stream isintroduced preferably upstream of all reaction zones receiving atransfer agent-poor stream. Similarly the monomer-rich recycled streamfrom at least one of the separators is advantageously supplied to anextent exceeding 75% by volume to one or more reaction zones upstream ofat least one downstream reaction zone. Suitably a recycled monomer-richfeed is supplied to an extent of from 75 to 100% of its volume to areaction zone upstream of all other reaction zones spaced lengthwisealong the tubular reactor.

A particular chain terminating transfer agent contemplated maypredominantly be propionaldehyde. Its effects may be focused on one ormore reaction zones. Propionaldehyde may cause a linear polymer to beformed, which can act as the platform for added polymerization indownstream polymerization reaction zones and higher conversion therein,without materially upsetting product quality. The chain terminatingtransfer agent may be added in such amounts in such a way that theconcentration of chain terminating transfer agent at the outlet of thetube is at least 50% less than that at the uppermost upstream reactionzone.

While in a conventional process using an almost homogeneous transferagent distribution, the process is run so as to convert as much of thepolymer in the first reaction zone as possible, it has been surprisinglyfound that advantageous operation results by increasing the proportionof transfer agent further by reducing the amount of monomer suppliedupstream, particularly the first upstream reaction zone. The mol % ofmonomer fed in the stream supplied to the reaction zone located furthestupstream may be less than that at least one reaction zone downstreamthereof.

By using a chain terminating transfer agent which gives by and large alinear polyethylene chain backbone, the process may not have to bemanipulated to obtain desired levels of branching and branching may beprovided by a transfer agent selected for that purpose. Such chaintransfer agents may incorporate poorly such as propylene or butene-1 butpreferably are selected to incorporate well, such as methylmethacrylate, as noted supra. In this way, the monomer rich part of therecycle contains relatively low levels of chain forming transfer agentsfor introduction in the upstream reaction zones.

The chain branch forming transfer agent may be fed predominantly toreaction zone or zones downstream of an upstream reaction zone.

Although the present invention has been described in considerable detailwith reference to certain preferred versions thereof, other versions arepossible. For example, while polyethylenes are discussed, otherpolyolefins are contemplated. Therefore, the spirit and scope of theappended claims should not be limited to the description of thepreferred versions contained herein.

What is claimed is:
 1. A tubular polymerization reactor apparatuscomprising: a) a source of fresh monomer; b) first and second compressorstages for compressing monomer; c) a reactor tube; d) multiple feeds,spaced lengthwise along the reactor tube for supplying monomer to thereactor; e) multiple free-radical or catalyst injection positions spacedlengthwise along the tubular reactor for causing monomer to be convertedinto polymer inside the tubular reactor; f) separators for receiving amonomer-polymer mixture from the reactor tube and separating saidmixture into a volatile monomer-rich phase and molten polymerizationphase; g) conduits for recycling the monomer-rich phase to the firstand/or second compressor stages for recycling unreacted monomer to thereactor tube; and h) a source of transfer agent for modifying themolecular weight of the polymer for compression and feeding to thereactor tube; wherein compressor means is provided for compressing atransfer agent rich stream separately from a transfer agent-poor monomerstream and means is provided for feeding the compressed transfer agentrich stream to a polymerization reaction zone upstream of at least onereaction zone receiving the transfer agent-poor stream containing lessthan 30 wt. % of the transfer agent relative to the transfer agent richstream.
 2. The reactor apparatus of claim 1, wherein compressor meanscomprises a further compressor stage for an initial compression of thetransfer agent obtained from the source of transfer agent, and a sectionof the second compressor stage, connected to the outlet of the furthercompressor stage for raising the gas stream containing transfer agent toa pressure suitable for supplying to the reactor, said section beingoptionally used additionally for compressing part of the monomerobtained from the outlet of the first compressor stage, said transferagent being passed through conduits segregated from another section ofthe second compressor stage which is used for compressing the transferagent-poor monomer stream.
 3. The reactor apparatus of claim 1, whereinthe transfer agent rich stream is connected to be introduced upstream ofall reaction zones receiving a transfer agent-poor stream.
 4. Thereactor apparatus of claim 3, wherein the monomer-rich recycled streamfrom at least one of the separators is connected to be supplied to anextent exceeding 75% by volume to one or more reaction zones upstream ofat least one downstream reaction zone.
 5. The reactor apparatus of claim4, wherein a recycled monomer-rich feed is connected to be supplied toan extent of from 75 to 100% of its volume to a reaction zone upstreamof all other reaction zones spaced lengthwise along the tubular reactor.6. The reactor apparatus of claim 2, wherein the transfer agent isconnected to be passed from the further compressor stage and combinedwith the recycled monomer-rich stream for compression in the segregatedsection of the second compressor stage.
 7. The reactor apparatus ofclaim 6, wherein a pair of sources of transfer agents are arranged forconnection to different reaction zones.